Priority is accorded to the preparation of (necessarily solvated) crystals of the low-spin carbonyl-1-alkylimidazoleiron(II) and dioxygen-1-aklylimidazoleiron(II) derivatives of the "picket fence" porphyrin (meso-tetra alpha, alpha, alpha, alpha-o-pivalamidophenyl)-porphine), H2(TpivalPP), that will provide X-ray data of the caliber required for substantially more quantitative determinations of structure than those reported heretofore (cf., J.L. Hoard in "Porphyrins and Metalloporphyrins", K.M. Smith, Ed., Elsevier Scientific Publishing Co., Amsterdam (1975), pp. 366-368; J.P. Collman et al., Proc. Natl. Acad. Sci. U.S.A., 71, 1326 (1974)). The joint use of a relatively nonvolatile aromatic solvent and a bulky 1-alkylimidazole is contemplated to this end. Although disorder in the orientation of the bent Fe-O-O linkage in the (O2)Fe(1-AlkIm)-TpivalPP) molecule will not be eliminated, it should be feasible to determine whether the superoxide or peroxide formulation is the better approximation for the complexed dioxygen entity. Other preparations which, if successful, are to be followed by structure determinations include: (1) A fluoro-1-alkylimidazoleiron(III) derivative of H2(TpivalPP) as a model system for fluoromethemoglobin. (2) Five-coordinate 2,6-dimethylpyridineiron(II) and 2,6-dimethylpyridineiron(III) metalloporphyrins to determine the stretch in the axial bond systems that is required by steric interactions. BIBLIOGRAPHIC REFERENCES: J.L. Hoard, "Stereochemistry of Porphyrins and Metalloporphyrins", Chapter 8 (pp. 317-380) in "Porphyrins and Metalloporphyrins", Kevin M. Smith, Ed., Elsevier Scientific Publishing Company, Amsterdam, The Netherlands (October, 1975). Five off-prints of this critical review are submitted under separate cover. J.P. Collman, J.L. Hoard, Nancy Kim, G. Lang, and C.A. Reed, "Synthesis, Stereochemistry, and Structure-Related Properties of alpha,beta,gamma,delta-Tetraphenylporphinatoiron(II)", J. Amer. Chem. Soc., 97, 2676 (1975).